The present section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present disclosure that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the following, a picture (sometimes called an image or frame in prior art) contains one or several arrays of samples (pixel values) in a specific picture/video format which specifies all information relative to the pixel values of a picture (or a video) and all information which may be used by a display and/or a decoding device to visualize and/or decode a picture (or video). A picture comprises at least one component, in the shape of a first array of samples, usually a luma (or luminance) component, and, possibly, at least one other component, in the shape of at least one other array of samples, usually a color component.
Low-Dynamic-Range pictures (LDR pictures) are pictures whose luma samples are represented with a limited number of bits (most often 8 or 10). This limited representation does not allow correct rendering of small signal variations, in particular in dark and bright luminance ranges. In high-dynamic range pictures (HDR pictures), the signal representation is extended in order to maintain a high accuracy of the signal over its entire range. In HDR pictures, luma samples are usually represented in floating-point format (either 32-bit or 16-bit for each component, namely float or half-float), the most popular format being openEXR half-float format (16-bit per RGB component, i.e. 48 bits per sample) or in integers with a long representation, typically at least 16 bits.
A dual modulation scheme is a typical approach for encoding an input HDR picture in a bitstream and for obtaining a decoded version of the input HDR picture by decoding the bitstream at least partially. Its principle is to obtain an illumination picture (also called illumination map or backlight picture) from the input HDR picture. A residual picture is then obtained by dividing the input HDR picture by the illumination picture and both the illumination picture (or illumination data representing the illumination picture) and the residual picture are then directly encoded.
Encoding an input HDR picture using this approach leads to encode two components: a residual picture (called the LDR picture in the following), which may be a viewable picture, and illumination picture (or illumination data representing the illumination picture). A decoded version of the LDR picture may then be obtained by directly decoding the bitstream at least partially, and a decoded version of the input HDR picture may also be obtained by multiplying the decoded version of the LDR picture by a decoded version of the illumination picture obtained by decoding the bitstream at least partially (or from illumination data obtained by decoding the bitstream at least partially).
The LDR and illumination pictures may have different input picture/video formats (YUV, RGB, XYZ, . . . ) and have not necessary the same format. For example, the illumination picture may be monochrome and the LDR picture format may be YUV or RGB.
The picture/video format of the decoded version of the input LDR picture, called the output LDR format in the following, and the picture/video format of the decoded version of the input HDR picture, called the output HDR format in the following, may be the same as the formats of the input LDR and HDR pictures, respectively called the input LDR format and the input HDR format in the following.
However, the input formats and the output formats are usually not the same because it is advantageous that the output formats be adapted to specific conditions. For example, an output LDR or HDR format may be adapted to some targeted specific displays, ambient conditions for the display of the decoded pictures or user preferences. Such adaptions of the output formats increase the visual quality of the decoded version of the pictures over said targeted specific displays because the encoding/decoding scheme optimizes the distribution of a given bit rate in order to reach the better visual quality for said targeted specific display.
The problem to be solved by the disclosure is that both a LDR or a HDR picture which may be decoded from a same bitstream shall conform to a specific output LDR or HDR format which is different from the input LDR or HDR format.
In a conventional encoding/decoding scheme adapted for encoding a single picture (or single video), the picture/video format of the input picture is signaled in the bistream using a syntax element: a so-called ‘video usability information’ (VUI) as defined, for example, in the HEVC recommendation (“High Efficiency Video Coding”, SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS, Recommendation ITU-T H.265, Telecommunication Standardization Sector of ITU, April 2013) or H264/AVC recommendation (“Advanced video coding for generic audiovisual Services”, SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS, Recommendation ITU-T H.264, Telecommunication Standardization Sector of ITU, February 2014)).
The picture/video format of the decoded version of the input picture is then the picture/video format defined by the VUI, i.e. the same as the picture/format format of the input picture.
Moreover, because such a conventional encoding/decoding scheme allows only a single VUI for signaling the input picture/video format of the picture to be encoded, it is not well-adapted for signaling two picture/video outputs formats in a same bitstream, each relative to a picture which may be decoded from the bitstream.